The present invention relates to a scroll fluid machine such as a scroll compressor, a scroll expander, a scroll vacuum pump, a scroll blower, etc. for handling compressive gas or liquid such as refrigerants, and more particularly to a scroll fluid machine suited for maintaining hermetic sealing in the axial direction by applying fluid pressure on the back side of an orbiting scroll.
Hitherto, it is known that, in a scroll fluid machine provided with back a pressure port (back pressure passage) from a compression chamber to the back pressure chamber of an orbiting scroll for keeping the hermetic sealing in the axial direction by pressing the orbiting scroll against a fixed scroll, a back pressure chamber side opening is opened only when the pressure of the compression chamber becomes approximately equal to that of the back pressure chamber in order to reduce power loss accompanying the flowing of fluid in and out of the back pressure port in a wide range of rotation speed. Such a technique is disclosed in, for instance, JP-A-H02-130284.